1. Field of the Invention
The present invention relates to an image fixing apparatus to be mounted on an image forming apparatus such as a copying apparatus or a printer, and a fixing rotary member for use in such image fixing apparatus.
2. Related Background Art
In a copying apparatus or a printer of an electrophotographic method, there is provided a fixing apparatus for heat fixing a toner image formed on a recording material. The fixing apparatus is available in various types such as a heat roller type for heat fixation by nipping and conveying the recording material with a fixing roller heated by an internally provided halogen lamp and a pressure roller, an on-demand type (also called film heating type) in which a ceramic heater is contacted with an internal surface of a flexible sleeve (fixing film or fixing belt) based on a heat-resistant resin or a metal and heats the recording material through the flexible sleeve, an electromagnetic induction heating type in which a rotary member itself in contact with the recording material generates heat and, in any type, a rotary member such as a fixing roller or a flexible sleeve is usually provided, on the surface thereof, with a releasing layer. Such releasing layer suppresses an offsetting of the toner to the rotary member. The releasing layer is required to have a heat resistant property and a releasing property, and such properties can be satisfied by fluorinated resins such as polytetrafluoroethylene (hereinafter also represented as PTFE), a perfluoroalkyl vinyl ether-tetrafluoroethylene copolymer (hereinafter also represented as PFA), and a hexafluoropropylene-tetrafluoroethylene copolymer (hereinafter also represented as FEP), which are widely employed for the releasing layer of the fixing rotary member.
Though the fluorinated resin has excellent heat-resistance property and releasing property and is commonly employed in the releasing layer of the fixing rotary member, it has a relatively high hardness because of its high melting point (for example PFA has a melting point of about 300° C.). In order to achieve satisfactory fixation of the toner, it is necessary, In addition to appropriate heating and pressurizing on the toner, to wrap up the toner by the fixing rotary member. In order to obtain a wrapping state for the toner, the fixing rotary member is required to have an approximate elasticity, and the fluorinated resin, having a relatively high hardness as mentioned above, is required to be provided thin in case of use as the releasing layer of the fixing rotary member.
However, an excessively thin releasing layer has a drawback of being easily broken, and is naturally insufficient in durability. Also a releasing layer of even an appropriate thickness is subjected to a loss in service life, not only by a long-term deterioration such as a surface roughening of the releasing layer during long-term use, but also by damage on the releasing layer by a separating claw for separating the recording material from the fixing rotary member or by damage on the releasing layer by the recording material when it is jammed In the vicinity of the fixing apparatus. The damage on the releasing layer by these factors is transferred onto the toner image at the fixing step, thereby deteriorating the image quality. Consequently, the damaged fixing rotary member has to be replaced.
On the other hand, in recent image forming apparatus, particularly in full-color image forming apparatus, there is required a function of varying a gloss (luster) of an image outputted from the apparatus. For varying the gloss in the fixing apparatus, there is known a method of suitably regulating a fixing temperature, a process speed (fixing speed), and a pressure etc. (for example Japanese Patent Application Laid-open Nos. H9-305058 and 2001-22219)
In a full-color image forming apparatus, there is required a satisfactory color reproducibility in the output image, and, for improving the color reproducibility, there are required conditions of a stable toner fusion state, an improved transparency of the toner image after fixation, and a sufficient mixing of toner. These conditions are not limited to a full-color image but also applicable to a monochromatic image.
However, a gloss control by varying the fixing temperature or the fixing speed as described in Japanese Patent Application Laid-open No. H9-305058 results in a change in the toner fusion state. Stated differently, a change in the surface condition of the toner image can regulate the gloss but also varies the fusion state not only on the surface but also inside the toner layer. For example, an increase in the gloss progresses the fusion inside the toner layer (for example the transparency of the 25 reS1n itself of the toner layer is elevated, and the color mixing of the toner layer is progressed). On the other hand, a decrease in the gloss suppresses the fusion inside the toner layer (for example the transparency of the resin of the toner layer is not elevated, and the color mixing of the toner layer is not progressed).
In order to increase the variation range of the gloss by the method described in Japanese Patent Application Laid-open No. H9-305058, it is necessary to increase a regulation range of the fixing temperature and the fixing speed, but, in such case, the toner layer shows a large difference in the fusion state between a high-gloss state and a low-gloss state, and the color reproducibility has to be sacrificed since a cold offset limit is present at the low gloss side and a hot offset limit is present at the high gloss side. On the other hand, in order not to sacrifice the color reproducibility, the regulation range for the fixing temperature and the fixing speed has to be made smaller. Therefore, in this method, it is in fact not possible to increase the regulation range of the gloss control.
On the other hand, Japanese Patent Application Laid-open No. 2001-22219 describes a method for controlling the gloss by varying a pressure. However, a method of varying the gloss by regulating the pressure also affects the transparency of the resin itself of the toner layer, as a press-out level of air layers present between the toner particles present in the toner layer varies by the pressure. Also depending on the type of the recording material, a toner penetration between fibers of paper is also influenced. For example, in case of obtaining a low-gloss image, there is selected a low pressure to reduce the toner penetration into the paper fibers, while, in a case of obtaining a high-gloss image, there is selected a high pressure to increase the toner penetration into the paper fibers, so that the color reproducibility is sacrificed. In order not to sacrifice the color reproducibility, the regulation range for the pressure has to be made smaller. Therefore, also in this method, it is in fact not possible to increase the regulation range of the gloss control.
Also it is known that a surface condition of the fixing rotary member in contact with the toner image influences the gloss, but, in order to vary the gloss of an image, it is necessary to employ, each time, a fixing rotary member of a different surface condition or a fixing apparatus having such fixing rotary member, and the gloss cannot be controlled in practice.
As explained in the foregoing, the gloss control methods proposed conventionally are unable to achieve a color reproducibility of an image and a gloss control of a large regulation range at the same time.